JP7299981B2 - Digital halftoning with clumped microdots - Google Patents

Description

Description The present invention relates to the field of digital halftoning methods, particularly those used for printing images on lithographic or flexographic printing presses.

Presses and digital printers cannot change the amount of ink or toner applied to a particular image area except through digital halftoning, also called dithering or screening. Binary digital halftoning renders the illusion of a continuous tone image (continuous tone image) by means of a halftone raster image (also called a "screen") made up of pixels (also called "microdots"). ) method, where pixels are either "on" (image pixels) or "off" (non-image pixels), i.e. corresponding to image and background regions of the image, respectively.

These pixels are generally arranged in a so-called halftone cell as a grid. The ratio of image pixels to non-image pixels in the halftone cell determines the relative image density and also determines the dot rate of the halftone cell. A halftone cell in which half of the pixels are image pixels and half are non-image pixels has a relative image density of 50%. Halftone cells representing light areas of the image (white areas) have an image density of less than 50%, and halftone cells with dark areas of the image (shadows) have an image density of greater than 50%. have.

Digital halftoning is a well-known method, which is described, for example, in Non-Patent Document 1, Section 5 of which on "Clustered-dot ordered dither", converts continuous-tone images to Background for the present invention involving the use of threshold arrays for rendering. Another overview of digital halftoning is disclosed in Non-Patent Document 2, which also describes multilevel digital halftoning.

The most widely used screening technique creates so-called amplitude modulated (AM) screens. AM screens consist of halftone cells, in which image pixels are grouped into clusters, which are called "halftone dots", not to be confused with the aforementioned microdots (=pixels). or "AM dots". AM screening achieves higher relative image densities by growing the size of the dot mass. When printing an AM image, for example using a printing press or a digital ink jet printer, each AM dot also corresponds to a quantity of ink, called a blob, which is pressed or jetted onto the substrate to be printed, Dried and/or cured. The thickness of the blob, whether wet-on-wet or wet-on (semi-)dry, as well as on the substrate, especially when multiple inks (color choices) are to be printed on top of each other Ink spreading determined by local (de)wetting of the substrate and/or ink absorption by the substrate causes the printed blob to be locally uncontrolled. As a result, the printed image may exhibit noise, and this problem is often dependent on the nature of the substrate.

Such problems can be addressed by other screening methods such as FM (Frequency Modulation) screening or methods involving error diffusion. In both of these methods, the microdots are more or less irregularly spread over the image rather than clustered in clumps. The local image density is modulated by the microdot frequency. However, these methods also suffer from other problems such as print stability, poor smoothness of flat tones, higher dot gain and higher wear of the printing plate in long press runs. characterized by

Hybrid screening methods are available that combine both methods to obtain the advantages of both AM and FM. However, said screening methods involve the use of multiple threshold arrays to represent continuous tone images, such as a threshold array using the FM method in bright areas, a threshold array using the AM method in midtones. and requires more memory space to store a separate threshold array using the FM method in the dark. Furthermore, transitions from one threshold array to another can create density jumps in the printed image, whereby calibration of the screening method also requires longer service times than AM and FM. become.

US Pat. No. 5,300,009 discloses a method for controlling the thickness of ink blobs in AM halftone areas of a printing plate or intermediate image carrier on a digital printing press. This method produces a raster image of regularly tiled halftone cells in which image pixels are arranged to form concentric rings, which surround areas of non-image pixels. As a result, the extent to which ink can spread within the cell is kept limited, because as soon as the enclosed portion is filled, further spreading of ink within the dot is not possible.

The latter problem is solved by co-pending application US Pat. the spiral halftone dots are (i) image pixels arranged as a first arc or as a plurality of arcs taken together representing a first spiral and (ii) as a second arc or taken together as a second spiral. It comprises non-image pixels arranged as a plurality of arcs representing two spirals. Compared to the concentric rings of US Pat. No. 5,430,000, the spiral dot comprises a longer, preferably open ink channel defined by a second arc or spiral, which allows better spreading of the ink.

US Pat. No. 5,300,001 discloses a digital halftone screening method in which conventional halftone dots are divided into discrete blobs by a dot function, and pixels are switched on or off within blobs by the blob function. This method involves complex mathematics and thus cannot be implemented in a wide installed base of prepress systems with limited data memory and/or processing power.

U.S. Application Publication No. 2007/0002384 International Application No. PCT/EP2018/079011 EP 346116

"Digital Halftoning", MI Press, 1987, ISBN 0-262-21009-6 "Recent trends in digital halftoning", Proc. SPIE 2949, Imaging Sciences and Display Technologies, (7 February 1997); doi: 10.1117/12.266335

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for reproducing continuous tone images on a printing press by means of halftone raster images having similar advantages to raster images comprising spiral dots disclosed in co-pending application WO 2005/020003. To provide a method that enables ink savings in lithographic or flexographic printing processes and that can be implemented in a wide installed base of prepress computer systems with limited amounts of memory and/or processing power.

This problem is solved by the method defined in claim 1, wherein a single threshold array is used to represent the continuous tone image, i.e. the continuous tone image is regularly laid out consisting of a grid of image pixels and non-image pixels. Convert to a halftone raster image comprising the halftone cells that were erased. When a raster image is exposed on the coating of a printing plate precursor, the image pixels correspond to the printing (ink-accepting) areas of the plate, while the non-image pixels correspond to the non-printing areas of the plate. In at least a portion of the halftone cell, the image pixels are arranged in multiple image clusters, defined herein as groups of more than four adjacent pixels spaced apart from each other. Image blobs allow a print of the same image density to be obtained with less ink than is used in conventional screens where the image pixels are grouped into one blob.

FIG. 1 shows six halftone cells in the upper half of the figure, each consisting of a 16×16 grid of pixels. These halftone cells were created with the same threshold array by the method of the present invention. The threshold array is the matrix shown in the bottom half of the drawing. Each halftone cell represents a different relative image density as indicated by the dot percentage shown below each halftone cell. Image pixels are represented in the drawings by black areas, while non-image pixels are represented by white areas. 2-11 differ from FIG. 1 only in that a different threshold array is used in each figure. FIG. 12 shows six paved halftone cells, each comprising four groups of 3×3 image pixels. Figures 13 and 14 show six overlaid halftone cells, identical to the 70% halftone cells in Figures 3 and 4, respectively. Figure 15 shows a halftone cell comprising three image blobs denoted by dashed lines AC; It does not follow the definition of an image block according to the invention. FIG. 16 shows in the top half of the drawing six halftone cells each consisting of a 16×16 grid of pixels, and in the bottom half the threshold array used to create these cells. The array thresholds range from 1 to 16, in contrast to the values in FIGS. 1-11, which range from 1 to 256. FIG.

Description of the preferred embodiment

Raster Image The raster image produced by the method of the present invention is suitable for representing a continuous tone image (CT), ie it creates the illusion of a continuous tone image (CT) on the printed copy. This requirement is that the number of halftone cells arranged next to each other per unit of length in the direction that produces the screen frequency or maximum is 40 lines per inch (LPI; 15.7 lines/cm ), more preferably higher than 60 LPI (23.6 lines/cm), and most preferably higher than 100 LPI (39.4 lines/cm). If the screen frequency is lower than 40 LPI, halftone dots become visible at a viewing distance, also called reading distance, which is approximately 20 cm. Such low screen frequencies are typically used in artistic screening, which is used for decorative purposes such as patterned illustrations where individual dots are intended to be visible to the naked eye. Raster images in which the halftone dots are clearly visible at viewing distance are therefore not suitable for the method of the present invention.

The aforementioned screen frequency defines the spatial frequency of the halftone cells contained in the raster image. As mentioned above, a halftone cell whose rotation is (on its
turn) consists of a grid of pixels and their spatial frequency, called resolution, is expressed as dots per inch (DPI) or pixels per inch (PPI). When a raster image is written, for example, on film or a printing plate with a scanning laser, the pixels are also called laser dots, and the resolution refers to the number of rays per inch. Raster images produced by the method of the present invention preferably have a resolution higher than 600 DPI, more preferably higher than 1200 DPI. Higher resolutions up to 9600 DPI can also be used for printing security features, for example.

Since the printing plate can only transfer a single color, it is clear that the halftone raster image used in the method of the present invention is a single color image, which is one of the four primaries in a multicolor printing process, e.g. CMYK printing. can represent a choice of

The quality of the printed image is preferably checked with a color density measurement. The color density values can then be used as input parameters for a prepress computer system that creates a raster image so that the quality of the printed image continues to improve on press runs and/or increases on subsequent press runs. Adjusted to save ink.

Halftone Cells The raster image produced by the method of the present invention comprises regularly spaced halftone cells. The cells can be laid out along a triangular, square or hexagonal grid, and preferably a square grid. Figures 13 and 14 each show an example of a preferred embodiment in which six halftone cells are laid out as a square grid.

The halftone cell itself also consists of a grid, more specifically a grid of pixels, which can be image pixels or non-image pixels. These pixels preferably have the shape of regular polygons or convex polygons, such as triangles, squares, rectangles, rhombuses or hexagons. The drawings show examples of preferred embodiments, where the halftone cells consist of a grid of square pixels.

A raster image produced by the method of the present invention comprises halftone cells having two or more image blobs, ie groups of more than four adjacent image pixels spaced apart from each other. In a more preferred embodiment, the raster image comprises halftone cells having more than 2 image chunks, such as at least 3 or 4 image chunks, more preferably at least 5 and most preferably at least 6 image chunks.

In the above definition of "image blob", image pixels are considered adjacent if they share at least one edge of a polygon. FIG. 15 shows three such image chunks: chunk A consisting of 7 contiguous image pixels; chunk B consisting of 6 contiguous image pixels; and chunk C consisting of 5 contiguous image pixels. Image pixels d, e and f touch another image pixel, but only at the corners of the square; these pixels do not share edges and are not considered neighbors as defined above. . Groups of 4 or less image pixels, such as group h, do not constitute an image block according to the definition above.

FIG. 12 illustrates a further refinement of the image blob definition used herein. When halftone cells are laid, a group of image pixels in one halftone cell can join another group of image pixels in another halftone cell at one or more adjacent edges. FIG. 12 shows an example where groups of image pixels labeled 3a lead to groups labeled 3b. According to the present invention, such groups are not considered separate chunks, but together represent a chunk. The resulting halftone cells shown in FIG. 12 contain only three image blobs each.

In contrast to conventional AM dots representing the same dot rate, the image blobs used in the present invention allow printed images of the same image density to be obtained with less ink. Although the reasons for this advantage are not fully understood, the inventors believe that presses run according to the method of the present invention are significantly more efficient than presses run with plates exposed on conventional AM screens of the same original. A systematic measure was taken to consume less ink. When compared to FM screens, it was observed that longer run times could be obtained because the agglomerates were larger than FM microdots and therefore less susceptible to wear on the press. FM screens consist of either a plurality of single image pixels or groups of 4 (2×2) image pixels, which degrade more easily than the image blobs used in the present invention.

Without being limited by the underlying mechanism, a halftone cell with agglomerated image pixels as described above has a higher density on the ink receptive area of the plate when compared to conventional AM halftone dots representing the same relative image density. It is currently envisioned that either a thinner adsorbed ink film and/or provide better spreading of the ink film into the empty (non-image) areas between the image blobs. Ink savings have been considered for different images and different plates. Savings of about 10% are often obtained. Since ink and paper are the major costs of a printer, even a few percent reduction in ink consumption represents a significant cost savings. Optimization of the raster image in relation to the printed substrate, for example by adjusting the number, size, shape and/or distribution of image blobs in the halftone cell, compared to conventional AM screening by 10 can lead to a further ink savings of 20%.

Lower ink consumption provides additional benefits resulting from, for example, faster drying and/or reduced energy consumption by drying equipment such as curing equipment and ovens. Rapid drying is particularly advantageous for printing uncoated plastic foils and newsprint. Better spreading of the ink also reduces ink set-off, ie the transfer of ink from one printed copy to the back side of another overlying copy, for example in the delivery tray of a press. Shine-through, also called print-through, by which the image is visible through the backside of the substrate, eg, thin, ink-absorbing substrates such as those used in printing newspapers, is also reduced. For all these reasons, it is particularly advantageous if the method of the invention is carried out in a complete press, ie a press that allows simultaneous printing on both sides of the substrate in one pass through the press.

In the preferred embodiment, the image blobs are locally concentrated rather than randomly distributed throughout the halftone cell, so that together the image blobs mimic conventional AM dots, and the method takes advantage of AM screens. maintain as much as possible. The image mass can be concentrated, for example, in a quarter fraction of the halftone cell. As a result, that quarter fraction represents a higher relative image density than other fractions of the halftone cell. More preferably, one quarter of the halftone cell has a relative image density that is at least twice the relative image density represented by the entire halftone cell. FIG. 5 represents an example of such an arrangement: 8×8 pixels around the center of the cell define a quadrant indicated by the dark line, which has a much higher image density than the entire cell. The higher density of image pixels need not necessarily be located near the center of the cell: FIGS. 3 and 4 show different local densities of image pixels, but these are shown in FIGS. 13 and 14 respectively. , these cells represent the same halftone image when laid next to each other.

The halftone raster images used in the present invention are combinations of different types of halftone cells, such as those described above in combination with conventional halftone cells, such as AM halftone cells, where all image pixels are grouped into one cluster. can contain halftone cells containing multiple image blobs. One or more portions of the image may be represented by FM screens. In a highly preferred aspect of the present invention, the subset of all halftone cells in the image representing the bright and midtones of the image, i.e. less than 50% relative image density, all comprise two or more image blobs as defined above. consists of halftone cells with In another aspect, only the portion of the halftone cell representing the bright and midtones of the image contains two or more image blobs. The portion may be as little as 5%. Preferably the portion is at least 10%, more preferably at least 25%, and even more preferably at least 50%.

In another preferred aspect of the invention, the image blobs are arranged along paths representing loops or spirals. The loop can be square, rectangular, ring or oval. Figure 10 represents a halftone cell with chunks arranged along a ring path, while Figures 6-9 represent halftone cells with chunks arranged along a spiral path, hereafter referred to as " It's called Spiral Dot. The image pixels and clumps represent a first spiral of ink-receiving areas and thereby define a second spiral of non-image pixels represented by the spaces between the windings of the first spiral. Without wishing to be bound by theory, it is assumed that ink transferred from the spiral dot onto a substrate such as a sheet of paper can flow into the second spiral, which acts as an ink receiving channel. ing. A spiral dot has the unique feature that all its non-image pixels touch each other, resulting in a higher degree of ink spreading than with the concentric rings of US Pat. In the midtones and light areas of the image, the second spiral is preferably open so that it can act as an ink channel to guide the ink out of the spiral dot: when the ink is pressed onto the substrate, the ink portion is As it is expelled from the second spiral and releases a small amount of ink at the outlet of the channel, it becomes visible in the enlargement of the printed image. Open channels allow more ink spreading, resulting in more ink savings and more rapid drying. A similar effect can be obtained with the halftone cell represented by FIG. 10, where the annulus has gaps between the masses that allow the suction of ink.

It should be clear to those skilled in the art that different spiral dots of the same overall size can be used to give the same dot coverage: a dot consisting of one turn of the first spiral of a certain thickness has a lower Gives the same coverage as a dot with more turns of the first spiral of thickness. The thickness of the first spiral may vary within the same dot, eg smaller in the center of the dot than at the edges. The spiral winding can be clockwise or counterclockwise, and both aspects can be combined within the same raster image. The starting angle of the first spiral is preferably the same for all spiral dots in the image at the center of the dot, or can be chosen randomly by a random number generator. Further details of suitable spiral dots are disclosed in co-pending application US Pat.

Threshold Array According to the present invention, the raster image is produced by a single threshold array. Digital halftoning with threshold arrays, sometimes called threshold matrices or threshold tiles, is well known in the art. When used for multicolor printing, the number of threshold arrays is preferably the same as the number of color channels in the original continuous tone image. Digital halftoning with a threshold array generally means that the original image is sampled into cells that map onto the threshold array. The local density values of the original image are then compared to each value in the array (adjusted, if necessary, so that the density range of the original image is equal to the range of the array). An output pixel is set to 0 if the original density value is lower than the pixel threshold. Otherwise, the output pixel is set to 1 if the density value equals or exceeds the threshold. These steps are repeated for all cells of the image.

The dimensions of the array, ie the number of pixels per halftone cell, can depend on various factors such as the resolution of the image setter and the desired quality of the printed image. The arrays are preferably arranged as squares (n×n dimensions) or rectangles (m×n, with m>n). FIGS. 1-6 represent examples of square threshold arrays having dimensions of 16×16 locations, where each location contains a particular range of threshold values. In these particular examples, the threshold range (1-256) equals the number of positions (16×16) in the array; in other embodiments, such as shown in FIG. 16, the range of values is lower than the number of positions There are possibilities, and then each individual value will occur at multiple locations in the array.

To create higher relative image densities, the threshold array is designed so that the number and/or size of image blobs rises to match the corresponding densities of the original image. For example, FIG. 1 shows a cell representing a relative image density of 25% using seven image blobs, whose size grows with increasing image density. Figure 3 shows a 10% cell with 5 image blobs, where the size and number of blobs grow at higher image densities. The image density of a halftone cell is grown by adding image pixels, while non-image pixels are preferably kept together with the non-image blobs, which are defined as groups of adjacent non-image pixels spaced apart from each other. By keeping the number of non-image blobs in the dark areas of the image as low as possible, the ink can spread to a greater extent when the non-image pixels are isolated or distributed in many blobs. In this way, ink can be saved even in the dark areas of the image.

In the preferred embodiment using spiral dots, the relative image density is determined by growing the length and/or thickness of the first spiral (which contains the image pixels) as shown in FIG. 6, or inside the second spiral. It can be augmented by interpolating image pixels; or it can be augmented by any combination of these methods. In the dark areas of the image, more image pixels are half-finished so that the ink channel formed by the second spiral (which contains the non-image pixels) shrinks and/or fades, as shown in FIGS. Added to the tone cell.

Printing Plate The printing plate used in the method of the present invention is obtained by exposing a halftone raster image to a light- or heat-sensitive material called a printing plate precursor. The plate master can be a positive or negative work. A positive printing plate precursor has a coating that accepts ink in the unexposed areas and is free of ink in the exposed areas after exposure and development. A negative plate accepts ink in the exposed areas and is devoid of ink in the unexposed areas. The image pixels shown in the drawing define the ink-receptive areas of the plate, which correspond to the exposed or unexposed areas of the negative or positive master, respectively.

The plate used in the method of the present invention is preferably a lithographic or flexographic printing plate. Lithographic printing plates generally convert a halftone raster image onto a printing plate precursor by means of a scanning laser, preferably a violet or near infrared laser, or another digitally modulated light source such as a digital mirror device, LCD or LED display. Obtained by exposure to a photosensitive or heat sensitive coating. After processing the exposed master with a suitable developer, a lithographic printing plate bearing the raster image of the invention is obtained. The plate can then be placed on a lithographic printing machine, preferably an offset press, where ink is supplied to the plate, which is then transferred to the substrate to be printed. Alternatively, the exposed master can be placed directly on the press, i.e. without any prior liquid treatment or other development, and the development of the image takes place in the ink and/or ink supplied to the plate at the start of the press. It can be caused by a fountain.

Flexographic plates are generally developed using UV lamps through a mask, which can often be a graphic film in contact with a photosensitive polymeric coating, or an in-situ mask overlying a photosensitive polymeric coating. Obtained by UV exposure of the molecular coating. The mask is preferably obtained by exposing a halftone raster image onto a film or in situ mask layer with a scanning laser, preferably a near-infrared layer.

The substrates on which the raster image can be printed can be of any kind, such as plastic films or foils, release liners, textiles, metals, glass, leather, hides, cotton and of course a wide variety of papers. It can be a substrate (lightweight paper, heavyweight paper, coated paper, uncoated paper, paperboard, corrugated board, etc.). The substrate can be a rigid work piece or a flexible sheet, roll or sleeve, or the like. Suitable flexible materials include, for example, paper, transparent foils, adhesive PVC, sheets, etc., which can have a thickness of less than 100 micrometers, and preferably less than 50 micrometers. Suitable rigid substrates include, for example, hardboard, PVC, carton, wood or ink-receivers, which can have a thickness of up to 2 centimeters, and more preferably up to 5 centimeters. be. The substrate may also be a flexible web material (eg paper, adhesive vinyl, textile, PVC, textile). A receptive layer, eg an ink receptive layer, can be applied on the substrate for excellent adhesion of the image to be reproduced on the substrate.

Claims (2)

A method for reproducing a continuous tone image on a printing press comprising:
(i) digital halftoning by converting the continuous tone image into a halftone raster image comprising regularly spaced halftone cells consisting of a grid of image pixels and non-image pixels;
(ii) exposing the halftone raster image onto the printing plate precursor;
(iii) developing said original plate thereby obtaining a printing plate;
(iv) placing the printing plate on the press;
(v) inking the printing plate and transferring the ink from the printing plate to the substrate;
comprising a step of
step (iii) can be performed after step (iv) instead of step (ii);
Step (i) is performed by sampling the continuous tone image into cells that map to a single threshold array, and wherein at least a portion of the halftone cells in the raster image contain two or more image blobs. wherein an image blob is defined as a group of more than 4 adjacent image pixels separated from each other ,
of all halftone cells wherein said portion has a relative image density of less than 50%, at least halftone cells of said portion comprise at least four image blobs;
image blobs in halftone cells of said portion are arranged along paths representing loops or spirals;
the aforementioned method. 2. The method of claim 1, wherein the portion of halftone cells has quarter divisions representing a relative image density that is at least twice the relative image density represented by the halftone cell as a whole.

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